Most ceramic bodies of alumina-zirconia are made by mixing finely ground or milled powders of alumina and zirconia, pressing to give a green body, and firing the green body to give a strong final body. Alumina from a wide variety of sources has been used. On the other hand, most manufactured alumina-zirconia abrasive grits are made by fusing zirconia and alumina together, then solidifying, often with special stratagems to produce very rapid cooling.
Most manufactured aluminous abrasive is made by fusing aluminum oxide with or without the addition of zirconia and with varying degrees of purity, depending upon the desired application.
Starting with the introduction of sintered bauxite in 1966 (U.S. Pat. No. 3,079,243), for use in heavy duty grinding of stainless steel, a minor but significant fraction of commercial aluminous abrasive began to be made by sintering rather than by fusion. Such sintered abrasive was used primarily for stainless steel snagging. As of 1960, a major difference between the fused and the sintered abrasives was that the sintered abrasive grits were microcrystalline, thus of enhanced toughness and fracture resistance. Although a small amount of fused alumina known prior to 1960 was rapidly quenched to produce polycrystalline grits, the majority of the fused alumina was in the form of grits in which each grit was made up of from 1 to 3 crystals (or parts of crystals) depending upon the grit size.
In the 1960's the fused alumina-zirconia alloy abrasives were introduced (U.S. Pat. No. 3,181,939). Such abrasives were, like the 1960 sintered abrasives, truly polycrystalline, thus having enhanced toughness and fracture resistance. Subsequently some patents were published covering sintered alumina zirconia, i.e. U.S. Pat. Nos. 3,481,723, 3,679,383 and 3,454,385.
In the early 1980's a new aluminous sintered abrasive made from alumina gel was introduced as taught in U.S. Pat. No. 4,314,827. Such abrasive contains about 5% of magnesia which forms a polycrystalline matrix phase surrounding cellular 5 to 10 micron sized alumina crystals. Such abrasive has been employed in coated abrasive belts and discs to provide high quality performance. The same patent also discloses a different aluminous abrasive, made by sintering alumina gel mixed with zirconia. Such abrasive has not been offered commercially and according to the data of the patent is inferior to the MgO containing abrasive.
Recently a different sintered abrasive made from alumina gel was described in European published patent application EP-152768 and U.S. Pat. No. 4,623,364. In these disclosures, very fine alpha alumina seed particles are added to an alpha alumina precursor gel to produce a fine and dense alpha alumina structure on sintering, without the need for MgO or other additions. The documents teach that such alumina may serve as a matrix for other ceramic materials such as zirconia. Other references that are related to use of seeded gels are:
The Effect of Cr3+ and FE3+ Ions on the Transformation of Different Aluminum Hydroxides to (alpha)-Al.sub.2 O.sub.3, T. Tsuchida et al., Thermochimica Acta, 64 (1983) 337-353. This article discusses the effects of additions of chromium and iron ions to boehmite, pseudo-boehmite and bayerite. The article notes on page 350 that the addition of Cr3+ ions to bayerite reduces the temperature at which the material converts to alpha alumina and explains this on the basis that the chromium ions crystallize as alpha Cr.sub.2 O.sub.3, and acts as active nuclei for the transformation of the alumina to alpha alumina. The article further states on page 352 and 353 that the iron ions crystallize as alpha Fe.sub.2 O.sub.3 and act as active nuclei for the transformation of the alumina to alpha alumina. The article says nothing about the crystal structure of the resulting alpha alumina. It bases its conclusion concerning nucleation on the fact that the additions of the chromium and iron ions act to reduce the temperature of transition to alpha alumina in the situations mentioned above.
Alpha alumina Formation in Al.sub.2 O.sub.3 Gels by F. W. Dynys and J. W. Halloran, "Proceedings of the International Conference, Ultrastructure Processing of Ceramics, Glasses and Composites" held Feb. 11-17, 1983 in Gainesville, Fla., Chapter 11, F. W. Dynys and J. W. Halloran, J. Wiley & Sons, Inc. 1984. This article discusses the effect of additions of Cr.sub.2 O.sub.3 and Fe.sub.2 O.sub.3 on colloidal suspensions of pseudoboehmite. The article notes that the additions of Cr.sub.2 O.sub.3 and Fe.sub.2 O.sub.3 in concentrations of greater than 2 wt. % enhanced the transformation rate to alpha alumina. The article notes that during annealing of the dried gel the alpha alumina colonies which formed in the material consisted of vermicular shaped particles separated by large elongated pores. As the gels are annealed for longer times, the vermicular alpha alumina rapidly grew and dominated the microstructure. It states on page 148 that the microstructures of gels doped with MgO, Cr.sub.2 O.sub.3, and Fe.sub.2 O.sub.3 were indistinguishable from those of undoped gels.
Alpha Alumina Formation in Alum-Derived Gamma Alumina, by F. W. Dynys and J. W. Halloran, Journal of the American Ceramic Society, Vol. 65 No. 9 p. 442. The article describes the results of research into the formation of alpha alumina in gamma alumina during sintering. The gamma alumina powder was treated in various ways such as die pressing and dry ball milling with alpha alumina milling media. The article observes on page 443 and 444 that the transformation from gamma to alpha alumina was more rapid for the gamma alumina powder which had been ball milled. To determine whether the alpha alumina milling debris acted as heterogeneous nuclei in the conversion of the gamma alumina to alpha, distilled water was milled for a period sufficient to produce an adequate amount of milling debris. The debris, which was largely alpha alumina, was collected by drying. The gamma alumina powder was doped with 1 percent of the milling debris and pressed. The weight versus time data for the doped powder was indistinguishable from that of the untreated powder which had no milling debris added, thus the article concludes that the debris had no affect on transformation and that the debris particles did not act as a seed for the crystallization of alpha alumina.
The article goes on to discuss on pages 446 and 447 the results of examining the annealed gamma alumina compacted powder which had been fired sufficiently to partially convert the gamma alumina to alpha. The article notes on page 447 that although the alpha particles are the same size as those in the unmilled powder the alpha colonies are much finer than those in the unmilled powder and that the nucleation frequency for the milled powder was dramatically increased by the milling operation.
The article goes on to state on the second column of page 447 that the mechanism by which the nucleation frequency is increased by ball milling is not obvious and that it had been demonstrated that the alpha alumina debris from the milling media is not responsible for the effect.
Influence of Cr and Fe on Formation of (alpha)-Al.sub.2 O.sub.3 from (gamma)-Al.sub.2 O.sub.3 by G. C. Bye and G. T. Simpkin. Journal of the American Ceramic Society, Vol. 57, No. 8, Pgs. 367-371, August 1974. The article reports the results of investigations into the influence of chromia and iron on the formation of alpha alumina. It notes on page 368 that the presence of Fe.sup.3+ ions in the gamma alumina accelerated the conversion to alpha alumina. On page 370, column 2 the article suggests that this is the case because the Fe.sup.3+ ion acts to decrease the crystallinity of the intermediate delta-Al.sub.2 O.sub.3 and possibly by the segregation of nuclei of alpha-Fe.sub.2 O.sub.3 but notes that the alpha-Fe.sub.2 O.sub.3 was not detected by X-ray diffraction. The article goes on the say that the formation of alpha-Al.sub.2 O.sub.3 involves either, (a) steps of sintering followed by synchro-shear, or, (b) a process of nucleation and growth, and states that the evidence supports the synchro-shear mechanism.
U.S. Pat. No. 3,387,957, issued June 11, 1968 to E. E. Howard describes a method for making alpha alumina abrasive grain by sintering calcined bauxite. The calcined bauxite is initially about 3/4 inch and finer in size and is milled in either a ball mill with alumina balls or a laboratory mill using either steel or alumina grinding media, to form a slurry. The slurry is dried to a cake and pulverized into agglomerates of finely ground microscopic particles. The pulverized material is then thoroughly mixed with a binder and extruded to produce solid cylindrical rods that are cut into grain sized pieces, dried and fired. Microscopic analysis of several grains showed that they were made up of microcrystalline particles on the order of five microns in size. The patent notes in the example described in column 6 that the calcined bauxite was milled for 100 hours in a rotary ball mill using cylindrical alumina balls and that the chemical composition of the material was changed somewhat as a result of the attrition of the alumina grinding media and the lining of the mill. The patent goes on to state that the composition following the milling was not significantly changed.
Since calcined bauxite is normally already alpha alumina, the sintering of the milled material described in the Howard patent does not convert a precursor alumina to alpha alumina but merely consolidates already existing alpha alumina. Additionally, the slurry formed by the milling described in the Howard patent is not a gel and the patent does not describe any sol gel process. There is no suggestion in the patent that the attrition of alpha alumina from the milling media played any role in the process of making the abrasive grain. The patent states in passing at column 2, line 63-66, that calcined bauxite is the preferred aluminous mineral source material although uncalcined or raw bauxite ore may also be used. The patent does not describe how uncalcined bauxite would be used in the process and whether or not any preliminary calcining step would be required. The method claims of the patent are limited to the use of calcined bauxite.
While, as indicated above, various alumina-zirconia sintered abrasives have been described in the patent literature, none have to date been described or produced which are superior to their fused counterparts in overall performance in the snagging of both stainless and carbon alloy steels. Fused alumina-zirconia abrasives generally have an average crystal size of less than 0.2 microns for both the alumina and the zirconia crystals of which they are composed.